When a battery of a first motor vehicle is flat (no power), it is known to connect a battery of a second motor vehicle across the flat battery in order to start the engine of the first vehicle. Such a technique is commonly referred to as jump starting. When the battery of a motor vehicle is not located in the engine compartment, but elsewhere in the vehicle, it has become common practice to provide jump start terminals in the engine compartment which can be connected to a battery of a second vehicle should the battery of the first vehicle go flat. If the second battery is incorrectly connected (that is, the wrong polarity) to the jump start terminals, electrical components in the first vehicle (especially diodes associated with the generator of the first vehicle) may be damaged.